Thanks to their improved performance in terms of output and power-to-weight and power-to-volume ratios, synchronous engines with permanent magnets are widely used today in the field of traction in automotive vehicles. Moreover, the availability of rare-earth permanent magnets on a large scale and under acceptable economic conditions makes the choice of such electric engines viable for new generations of automotive vehicles.
Such electric engines can be produced in a wide range of powers and speeds and will find applications in all-electric vehicles and in low CO2 vehicles of the types known as “mild-hybrid” and “full-hybrid”.
“Mild-hybrid” applications generally concern electric engines of the order of 8 to 10 KW, for example, an electric engine mounted at the front of a heat engine and coupled to it by a drive belt. It is possible with such an electric engine to reduce the thermal cubic capacity (engine downsizing) by providing torque electrical assistance which supplies auxiliary power especially when accelerating. Moreover, low-speed traction, for example in an urban environment, can also be provided by this same electric engine. Applications of the “full-hybrid” type generally concern engines of 30 to 50 KW for architectures of the series and/or parallel type with a more successful level of integration of the electric engine or engines in the vehicle's traction chain.
Among the known different synchronous engines with permanent magnets those of the flux concentration type are of particular interest owing to their excellent performance. In these flux concentration engines, the magnets are buried in the magnetic body of the rotor and arranged according to a roughly radial configuration.
In an automotive vehicle, an electric engine used in traction on all of the vehicle's circulation tasks is subject to variable conditions of speed and charge. A strategy of maximum torque control complemented by a defluxing strategy (also called “demagnetisation strategy”) to achieve the high speed zone seems to be a good solution for controlling the electric engine. However, defluxing in engines with permanent magnets presents a risk of demagnetisation of the magnets and even more so when the temperature of the magnets is high. Flux concentration engines, thanks to the arrangement of their magnets in the rotor, are particularly exposed to this risk of demagnetisation.
It is therefore desirable to propose synchronous rotating electrical machines with permanent magnets and flux concentration in which the risk of demagnetisation of the magnets is controlled or even totally eliminated.